Fix test_files_api_read_twice_from_one_download (#326)

## Changes
This integration test is failing (and likely wasn't passing when I
merged the original PR). This PR fixes it by unifying the codepaths for
`__enter__` and `read` to call `_open`, which consistently will check if
the response is closed and throw an exception if so.

## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->

- [ ] `make test` run locally
- [ ] `make fmt` applied
- [ ] relevant integration tests applied

Author: mgyucht

databricks/sdk/core.py

@@ -1147,8 +1147,14 @@ def __init__(self, response: requests.Response, chunk_size: Union[int, None] = N
         self._content = None
         self._chunk_size = chunk_size
 
+    def _open(self) -> None:
+        if self._closed:
+            raise ValueError("I/O operation on closed file")
+        if not self._content:
+            self._content = self._response.iter_content(chunk_size=self._chunk_size)
+
     def __enter__(self) -> BinaryIO:
-        self._content = self._response.iter_content(chunk_size=self._chunk_size)
+        self._open()
         return self
 
     def set_chunk_size(self, chunk_size: Union[int, None]) -> None:
@@ -1162,10 +1168,7 @@ def isatty(self) -> bool:
         return False
 
     def read(self, n: int = -1) -> bytes:
-        if self._closed is None:
-            raise ValueError("I/O operation on closed file")
-        if not self._content:
-            self._content = self._response.iter_content(chunk_size=self._chunk_size)
+        self._open()
         read_everything = n < 0
         remaining_bytes = n
         res = b''